It is known to provide a coating of a refractory oxide on a substrate by contacting the substrate with a sol of the refractory oxide followed by drying to convert the sol to a gel to give a gel-coated substrate, and optionally firing. For example, the specification of our U.K. Pat. No. 1,490,977 (corresponding to U.S. Pat. No. 3,957,692) describes, inter alia, contacting an aluminium bearing ferritic alloy substrate, either oxidised or unoxidised, with a boehmite sol, followed by drying to convert the sol to the corresponding gel and firing. Also, the specification of our West German OLS No. 2 647 702 (corresponding to U.S. patent application Ser. No. 733,152 Oct. 18, 1976) describes, inter alia, carrying out a similar process but using an alumina sol which has been made by dispersing in water an alumina prepared by flame hydrolysis. In each of the above instances, the alumina coatings, both in the gel and in the fired form, are particularly suitable as carriers of catalytically active material, such as a platinum group metal, in catalysts. The sols used in each of these instances comprise aggregated colloidal primary particles.